A light-emitting element having features such as thinness, lightweight, and rapid response is expected to be applied to flat panel displays of the next generation. In addition, it is said that a light-emitting device in which light-emitting elements are arranged in matrix is superior to conventional liquid crystal display devices in viewing angle and visibility.
A light-emitting element is formed by interposing a layer including a luminescent substance between a pair of electrodes (an anode and a cathode), and it is said that emission mechanism thereof is as follows: when a voltage is applied between both electrodes, holes injected from the anode and electrons injected from the cathode are recombined in a light-emitting layer in the layer including a luminescent substance, thereby forming a molecular exciton by recombination in an emission center, and energy is released to emit light when the molecular exciton returns to a ground state. By such a mechanism, such a light-emitting element is referred to as a current excitation type light-emitting element. A singlet excitation state and a triplet excitation state can be given as types of an excitation state formed by a luminescent substance. Light emission from a singlet excitation state is referred to as fluorescence and light emission from a triplet excitation state is referred to as phosphorescence.
Emission wavelength of a light-emitting element is determined by energy difference between a ground state and an excited state, that is, a band gap, of a light-emitting molecule included in the light-emitting element. Therefore, various emission colors can be obtained by devising a structure of the light-emitting molecule. By manufacturing a light-emitting device using light-emitting elements capable of emitting red light, blue light, and green light, which are the three primary colors of light, a full-color light-emitting device can be manufactured.
However, until now, there has been a problem in that it is difficult to realize a light-emitting element having high reliability and excellent color purity. As a result of recent development of materials, high reliability and excellent color purity of light-emitting elements for green and red have been achieved. However, in particular, high reliability and excellent color purity of a light-emitting element for blue has not been realized, and many researches have been done (for example, Reference 1: Japanese Published Patent Application No. 2004-75580).